1. Field of the Invention
The present invention relates to the installation of a hypodermic syringe and, more particularly, to a disposable safety hypodermic syringe.
2. Description of the Related Art
In order to prevent possible contamination after injection, safety hypodermic syringes are commonly used. A safety hypodermic syringe keeps the needle cannula concealed inside the barrel after its service. FIG. 1 illustrates a safety hypodermic syringe according to the prior art. According to this design, the safety hypodermic syringe comprises a barrel 60, an annulus connector 61 inserted into the inside of the barrel 60 and stopped inside the front end of the barrel 60, a needle hub 62 fastened to the connector 61 to hold a needle cannula outside the barrel 60, and a plunger 63 fitting the barrel 60. The plunger 63 has a front end fixedly mounted with a stopper, which has an arrowhead tip 64 for engaging the connector 61 for enabling the connector 61 and the needle hub 62 to be pulled backwards to the inside of the barrel 60 after the service of the safety hypodermic syringe. This structure of safety hypodermic syringe is still not satisfactory in function. When the plunger 63 moved forwards to force the liquid medicine 65 out of the barrel 60 through the needle cannula in the needle hub 62, the arrowhead tip 64 of the stopper is forced into engagement with the connector 61. At this time, the plunger 63 is prohibited from forward movement, and a residual amount of liquid medicine 65 left in the gap between the connector 61 and the stopper, resulting in an insufficient dosage give to the patient.
Therefore, it is desirable to have a safety hypodermic syringe that eliminates the aforesaid problem.
The present invention has been accomplished under the circumstances in view. It is therefore the main object of the present invention to provide a safety hypodermic syringe, which enables the full dosage of liquid medicine to be completely applied to the patient after its service. It is another object of the present invention to provide a safety hypodermic syringe, which enables the needle cannula to be concealed inside the barrel and the plunger separated from the barrel after its service.
To achieve these and other objects of the present invention, the safety hypodermic syringe comprises a body unit, the body unit comprising a barrel, a hollow cylindrical connector inserted into the barrel and stopped inside a front end of the barrel, and a needle hub fastened to the connector and holding a needle cannula outside the barrel; and a plunger unit, the plunger unit comprising a stepped plunger fitting the barrel, the stepped plunger having a front tip, a hollow stopper mounted on the stepped plunger around the front tip, and a flexible safety cap capped on the hollow stopper, the flexible safety cap having a locating groove of smoothly arched cross section extended around the periphery thereof; wherein the hollow cylindrical connector has a plurality of inside annular flanges adapted to secure the needle hub to the connector; the stepped plunger has an annular groove extended around the periphery thereof near the front tip; the hollow stopper has an inside annular flange forced into engagement with the annular groove of the stepped plunger; the front tip is forced through the flexible safety cap into engagement with the inside annular flanges of the hollow cylindrical connector when the plunger continuously forced forward after a forward stroke of the flexible safety cap and the hollow stopper with the plunger to expel a liquid medicine out of the barrel through the needle cannula, for enabling the needle hub and the needle cannula to be moved with the connector and the flexible safety cap and the hollow stopper backwards to the inside of the barrel upon a back stroke of the plunger; the front tip is breakable when the plunger biased by an external force after the connector has been moved with the plunger backwards to a rear end of the barrel.